1. Field of the Invention
The present invention relates to an optical element, and particularly to an optical element having an antireflection structure formed on the surface of the optical element that is formed by a press molding method, wherein the antireflection structure has a minute uneven structure whose average pitch is 400 nm or less.
2. Description of the Related Art
An optical element (lens) using a transparent medium (transparent member) such as a glass or plastic is generally subject to a surface treatment of forming an antireflection film on a light entrance surface to reduce a loss of transmitted light by a surface reflection. For example, a multi-layer film (so-called multi-coat) formed by stacking a plurality of dielectric thin films has been known as the antireflection film for visible light. The multi-layer film is formed by forming a metal oxide on the surface of the transparent member by a vacuum deposition or a sputtering method.
However, since the antireflection film formed by this process utilizes the interference of the reflected light generated at the surface and interface of each film, there is a problem that a sufficient antireflection property cannot be obtained for light other than the designed wavelength and designed incident angle.
As another antireflection method, there has been known a method for forming a minute uneven structure on the surface of the transparent member. When the pitch of the minute uneven structure is not greater than a wavelength, and has a pyramidal shape such as a circular cone or a square pyramid, the pitch becomes equivalent to a film whose refractive index gradually changes from air toward a substrate, so that an antireflection property excellent in wavelength band characteristic and incident angle characteristic can be obtained.
Japanese Patent Application Laid-Open No. 2007-283581 discusses a method in which a crown boron silicate glass is press-molded with the use of a molding die having a minute uneven structure on its surface, and the shape of the minute uneven structure is transferred onto the surface of an optical element to reduce the surface reflection.
Since the optical element formed by the technique discussed in Japanese Patent Application Laid-Open No. 2007-283581 has the antireflection structure including the minute uneven structure on its surface, it is expected to have an antireflection property excellent in a wavelength band characteristic and an incident angle characteristic.
Unlike an optical element having a spherical optical surface, an optical element having only an aspherical optical surface is difficult to form only by grinding. Therefore, a press molding method is generally used. However, a glass that has a low glass transition point (Tg) and used for the press molding is low in chemical durability, so that a problem as the optical element arises such as a surface deterioration generally called “tarnish” or “cloud” caused by a deposited substance. Further, when the minute uneven structure is formed on the surface of the glass, the surface area increases, with the result that the tarnish or cloud is more likely to occur than on a flat surface (on an ordinary lens surface).